1. Field of the Invention
This invention relates generally to a process for forming a silicon film, and more particularly to a process for forming an amorphous silicon film having excellent electrical and thermal properties.
2. Description of the Prior Art
Semiconductor material used heretofore for fabricating semiconductor devices are generally single crystal. A method of manufacturing such a semiconductor device comprises the steps of fabricating a single crystal semiconductor ingot by a pulling method, slicing the ingot in a sheet to obtain a wafer, and doping impurities in the wafer to form semiconductor elements or epitaxially growing predetermined semiconductor materials on the wafer by liquid or vapor phase epitaxy to form single crystal semiconductor layers.
The fabrication of the single crystal semiconductor requires several process, and also tremendous efforts must be exerted for processing the semiconductor materials in the single crystal. Furthermore, the single crystal ingot fabricated by the process explained hereinabove includes many crystal defects at the periphery thereof and can not be used as a material for manufacturing the semiconductor device. In addition, it is necessary to prepare lots of single crystal semiconductor materials to produce one semiconductor device, because the ingot must have a certain thickness so that it may be sliced in a wafer. Accordingly, the semiconductor device using the single crystal semiconductor material is disadvantageous in that the cost of the wafer or the device itself becomes expensive.
Recently, several attempts have been made to produce semiconductor devices without using single crystal semiconductor materials as a result of extensive researches as to the materials for fabricating these devices. One of these attempts is to produce amorphous substances by depositing component elements on a substrate by vapor evaporation or sputtering processes so that they may be used as a semiconductor material. If the amorphous substances can be used as the semiconductor material, it is possible to make use of inexpensive materials, such as, for example, glass or stainless steel, as a substrate. In addition, the amorphous substances have a large absorption coefficient. Accordingly, an optical device could be produced with less amount of the material and its manufacturing process could be remarkably simplified. Although the semiconductor device made of the amorphous substance is inferior in its characteristics as compared with those of the single crystal semiconductor device, the amorphous substance is appealing, because the device can be produced at a reduced cost and the manufacturing process can be simplified.
The single crystal substance has both long and short distance orders in its atom distribution, while the amorphous substance has the short distance order and does not have the long distance order. In other words, the amorphous substance consists of an unsaturated bond, namely, a dangling bond which lacks connection in covalent bond in several atom orders, and includes a number of dislocations. Thus, the electron state of the amorphous substance is in a band tail state which lacks a clear width in state density of the electrons and also in a mid gap state having deep localized level, which makes it difficult to effect doping for determining conductivity type of the semiconductor and to control valence electron.
In order to eliminate the dangling bond explained hereinabove, several attempts have been made. One of the attempts is to form the amorphous silicon film on a substrate by a glow discharge method which decomposes silicon hydride, such as, for example, monosilane (SiH.sub.4) gas, in high frequency electric field (several hundreds to several MH.sub.z).
FIG. 1 schematically illustrates a method of producing the amorphous silicon film. The amorphous silicon film is produced by introducing the SiH.sub.4 gas having a pressure of 0.1 Torr to several Torr in a vessel 2 which contains a substrate 1 and is wound by a high frequency coil 3, and supplying high frequency electric power to the high frequency coil 3 thereby to produce glow discharge around the substrate 1 and generate plasma 4 of SiH.sub.4. Silicon and hydrogen decomposed in the plasma 4 are accumulated on the substrate 1 which is heated at an appropriate temperature, for instance, 200.degree. C. to 350.degree. C., and an amorphous silicon 5 is formed on the substrate 1.
This glow discharge process is intended to eliminate the dangling bond by making the amorphous silicon 5 contain a large volume of hydrogen, thereby to dissolve the mid gap state, namely, the deep localized level between the band.
On the other hand, the band tail state can be diminished by creating a bond between silicon and silicon in the amorphous silicon 5 as much as possible. This can be done by increasing the heat temperature of the substrate 1 shown in FIG. 1. However, if the heat temperature is raised, hydrogen can not be injected into the amorphous silicon and the dangling bond can not be eliminated. Accordingly, in the glow discharge process, improvements in the band tail state can not be expected too much. Furthermore, the amorphous silicon produced by the glow discharge process is thermally unstable and variable in its electric characteristics, because it contains a large volume of hydrogen which is generally 10 to 20 atom % and permits hydrogen to escape if the substrate is heated up to the temperature of above 200.degree. C., thereby placing the amorphous silicon in the initial mid gap state.
This can be proved by measurement of optical band gap for regulating band gap of amorphous substance. The broken line (a) in FIG. 2 shows variation of the optical band gap when the amorphous silicon formed by the glow discharge process is annealed at a different temperature. As is apparent, the optical band gap decreases when the anneal temperature exceeds 200.degree. C., which results in variation of the electric characteristics. The broken line (a) in FIG. 3 shows conductivity of the amorphous silicon film at the respective anneal temperatures which is formed by the glow discharge process, and it can be recognized that the electric conductivity of the amorphous silicon film is variable depending upon the anneal temperature. The variation of the electric conductivity is resulted from the escape of hydrogen saturated in the dangling bond due to the heating, and the escape of hydrogen increases the mid gap state which is acted as an electron capture level and decreases the electric conductivity.
As explained hereinabove, the amorphous silicon film produced by the prior art process is defective, because it is thermally unstable and easy to deteriorate its electric characteristics. The instability of the conventional amorphous silicon film can be proved by an examination of bond structure of Si-H by infrared absorption measurement. In the amorphous silicon film formed by the conventional glow discharge process, a peak of absorption coefficient detected by the infrared absorption measurement exists at the points of both wave numbers 2000 cm.sup.-1 and 2100 cm.sup.-1 of incident infrared rays. When the absorption coefficient has the peak at the point of 2000 cm.sup.-1, it is recognized from the relation between infrared absorption characteristics and the bond structure which is already known in the art that the bond structure is in a monohydride state connecting one hydrogen atom to any one of the silicon atoms and eliminating the dangling bond as schemtically illustrated in the structural formula of FIG. 4. When the absorption coefficient has the peak at the point of the wave number 2100.sup.-1, the bond structure is in a dihydride state connecting two hydrogen atoms to any one of silicon atoms and eliminating the dangling bond as shown in FIG. 5. Thus, in the conventional amorphous silicon film, there are a plenty of the dihydride bond structures in addition to the monohydride bond structures. The dihydride state is considered to be unstable, because hydrogen easily escapes from the bond structure. The instability of the amorphous silicon film can be proved from the bond structure as explained hereinabove. Because of the instability of the characteristics of the amorphous silicon, it has not yet been put to practical use in spite of being recognized for its numerous advantages with respect to the manufacturing cost and the production process for the semiconductor device.